One method which is widely used to produce high modulus polymer sheets is the formation of fiber reinforced composites using, e.g. oriented polyethylene fibers in order to reinforce the polymer matrix. The manufacture of such composites is a complex operation and in particular requires careful mixing of the polymer and the fibers if the composite is to exhibit homogeneous mechanical properties.
There have been proposals to produce polymeric sheets by compression of networks of polymer fibers at elevated temperatures most notably in relation to thermotropic liquid crystal polymers. European Patent 354285 and U.S. Pat. No. 4,384,016 both describe processes in which fibers of a liquid crystal polymer are hot pressed to produce an oriented polymer sheet. European Patent Application 116845, describes a process in which a network of fibers of ultra-high molecular weight polyethylene are hot compressed to form polymer sheets. In the processes taught in this document the fibers are compressed and heated simultaneously. The products retain a significant proportion of the properties of the fibers in the direction in which the fibers are aligned but the mechanical properties of the products in the direction transverse to that in which the fibers are aligned is less than ideal. These processes are relatively unaffected by the choice of compaction temperature. The polymer fibers do not melt during the process.